[MapProxy] WMS 1.3.0 Extended layer capabilities

[Miloslav Kmeť]

Hi Oliver, 

I created on reuqest of my collegues patch for adding WMS 1.3.0 extended layer 
capabilities (Abstract, Attribution, MetadataURL...)

Please, take a look at 
https://bitbucket.org/milokmet/mapproxy/changeset/b32eb0a775d9

Would be fine, if this could be included into the main source tree.

This is example of YAML configuration covering extended capabilities elements 
for layers:

layers:
  - name: Layer1
    title: Layer 1
    sources: [osm_cache]
    md:
      abstract: Some abstract
      keyword_list:
        - vocabulary: Name of the vocabulary
          keywords:   [keyword1, keyword2]
        - vocabulary: Name of another vocabulary
          keywords:   [keyword1, keyword2]
        - keywords:   ["keywords without vocabulary"]
      attribution:
        title: My attribution title
        url:   http://some.url/
        logo:
          url:    http://some.url/logo.jpg
          width:  100
          height: 100
          format: image/jpeg
      identifier:
        - url:    http://some.url/
          name:   HKU1234
          value:  Some value
      metadata:
        - url:    http://some.url/
          type:   INSPIRE
          format: application/xml
        - url:    http://some.url/
          type:   ISO19115:2003
          format: application/xml
      data:
        - url:    http://some.url/datasets/test.shp
          format: application/octet-stream
        - url:    http://some.url/datasets/test.gml
          format: text/xml; subtype=gml/3.2.1
      feature_list:
        - url:    http://some.url/datasets/test.pdf
          format: application/pdf

I do not know, whether it is necessary to add this to config/spec.

Thank you for any discussion about the patch.
    
-- 
Miloslav Kmeť